"Hard-scattering" approach to very hindered magnetic-dipole transitions in quarkonium

TL;DR

This paper introduces a 'hard-scattering' approach to describe hindered magnetic dipole transitions in quarkonium, providing a better match to experimental data than traditional methods.

Contribution

It proposes a novel factorization formula based on a hard-scattering picture, applicable to hindered M1 transitions in quarkonium, with no free parameters and good agreement with measurements.

Findings

The formula accurately predicts the rate of  ext{Upsilon}(3S) o \u0016 ext{eta}_b+\u03b3

It reasonably accounts for other hindered M1 transition rates

Predictions for  ext{Upsilon}(4S) o \u0016 ext{eta}_b^{( ext{prime})}+\u03b3 are provided

Abstract

For a class of hindered magnetic dipole ($M1$) transition processes, such as $\Upsilon(3S)\to \eta_b+\gamma$ (the discovery channel of the $\eta_b$ meson), the emitted photon is rather energetic so that the traditional approaches based on multipole expansion may be invalidated. We propose that a "hard-scattering" picture, somewhat analogous to the pion electromagnetic form factor at large momentum transfer, may be more plausible to describe such types of transition processes. We work out a simple factorization formula at lowest order in the strong coupling constant, which involves convolution of the Schr\"odinger wave functions of quarkonia with a perturbatively calculable part induced by exchange of one semihard gluon between quark and antiquark. This formula, without any freely adjustable parameters, is found to agree with the measured rate of $\Upsilon(3S)\to \eta_b+\gamma$ rather well, and can also reasonably account for other recently measured hindered $M1$ transition rates. The branching fractions of $\Upsilon(4S)\to \eta_b^{(\prime)}+\gamma$ are also predicted.